Long Term Evolution (LTE) is a set of enhancements to the Universal Mobile Telecommunications System (UMTS) that supports high data rates, low latency, low implementation and operating costs, and a seamless connection to legacy wireless communication networks. LTE will improve spectral efficiency, in part by supporting flexible carrier bandwidths.
One major issue for LTE implementation is the lack of free spectral bands that are sufficiently wide to allow very high data throughput. In most potential markets, available frequency bands are fragmented, and a continuous band of, e.g., 100 MHz or more likely will not be available.
To address this issue for LTE it has been suggested that each transmission link set-up between a base station and user equipment (UE) should be able to allocate several sub-bands, distributed over a very large bandwidth, to form an aggregated spectrum. Furthermore, the locations of these sub-bands are generally not pre-determined. In particular, the standard LTE 3GPP release 10 will provide that typically, a UE will receive information from only one carrier—the anchor carrier—but can be scheduled by the network to receive information from additional carriers—called component carriers—when needed or allowed.
These component carriers can be located within the same frequency band as the anchor carrier, and/or in completely different bands simultaneously. When scheduled to receive additional component carriers, the receiver must continue to receive the anchor carrier without interruption. One implication of this requirement is that the RF LO used for down-converting the anchor carrier may not be allowed to change. This calls for additional RF LO generators to be used for the component carriers. However, having several independent RF LO synthesizers increases receiver complexity, cost, and power consumption, and presents additional challenges in high frequency design, such as effective shielding and isolation.